1. Field of the Invention
The present invention relates to an organic thin film transistor and a flat panel display apparatus comprising the organic thin film transistor, and, more particularly, to an organic thin film transistor wherein an organic semiconductor layer does not cause a coffee stain effect and does not have imperfect contact with source and drain electrodes and a flat panel display apparatus comprising the organic thin film transistor.
2. Discussion of Related Art
Since the development of polyacetylene, a conjugated organic polymer having semiconductor characteristics, organic transistors for functional electronic and optical devices have been widely and actively researched and developed due to the various advantages of such organic materials. For example, such organic materials can be synthesized using various methods and can be formed in an assortment of shapes such as fibers and films. In addition, such organic materials are flexible, conductive, and can be produced at low costs.
In a conventional silicon thin film transistor comprising a silicon semiconductor layer, high concentration impurity-doped source and drain regions are formed with a channel region therebetween in the semiconductor layer. A gate electrode, which is insulated from the semiconductor layer, is located in a region corresponding to the channel region. Source and drain electrodes are formed contacting the source and drain regions, respectively.
However, the manufacturing cost of the conventional silicon thin film transistor having the above-described structure is high, and the conventional silicon thin film transistor is easily broken by an external impact. In addition, since the silicon thin film transistor is fabricated at a high temperature of 300° C. or more, a plastic substrate cannot be used because it must be treated at low temperature.
In particular, thin film transistors are used as switching devices for controlling the operation of pixel electrodes and as driving devices for the pixels in flat panel display apparatuses such as a liquid crystal displays, electroluminescent display devices (ELD), etc. Recently, in order to satisfy demands for thin, small, flexible flat panel display apparatuses, usage of a substrate made of a plastic material, rather than conventional glass material, has often been attempted. However, the plastic substrate must be treated at a low temperature, not the high temperature described above. Therefore, it is very difficult to use the conventional silicon thin film transistor on plastic substrates.
However, these problems can be solved by using an organic layer as a semiconductor layer of a thin film transistor. Therefore, organic thin film transistors using an organic layer as their semiconductor layer have recently been widely studied.
However, in the organic thin film transistor, it is difficult to pattern the organic semiconductor layer. In addition, there is a problem with imperfect contact between the organic semiconductor layer and the source and/or drain electrodes.
When patterning the organic semiconductor layer of the organic thin film transistor, photolithography, which is a conventional patterning method, cannot be used. In particular, when the circuit comprises a plurality of organic thin film transistors, an organic semiconductor layer must be patterned so as to prevent cross talk between adjacent organic thin film transistors. However, when the organic semiconductor layer is patterned using conventional wet and/or dry etching processes that follow conventional patterning methods, the organic semiconductor layer is seriously damaged and cannot be used. Therefore, a method of forming an organic semiconductor layer using an inkjet printing method, which does not require an additional patterning process, has been suggested. According to this method, an organic semiconductor-forming material is dropped only in predetermined regions, thereby resulting in the pattern of the organic semiconductor layer without further processing. However, when the organic semiconductor layer is formed by such an inkjet printing method, it results in a “coffee stain effect.”
FIGS. 1 and 2 depict a schematic plan and a cross-sectional view, respectively, of an organic semiconductor layer with a coffee stain effect.
Referring to FIGS. 1 and 2, when an organic semiconductor layer forming material is dropped onto, for example, a substrate 1, to form an organic semiconductor layer 2 using an inkjet printing method, the thicknesses of the edge and center portions of the organic semiconductor layer 2 vary with time. The variation in the thickness of the organic semiconductor layer is large when the organic semiconductor layer has a non-circular shape, for example, a rectangular shape, etc. Further, when using an inkjet printing method to form the organic semiconductor layer, it is relatively easier to form the organic semiconductor layer in a circular shape. Thus, it is desirable to form the organic semiconductor layer in a circular shape.
FIG. 3 is a schematic plan view of a conventional organic thin film transistor having a circular organic semiconductor layer. FIG. 4 is a cross-sectional view of the conventional organic thin film transistor taken along line IV-IV of FIG. 3. The organic thin film transistor shown in FIGS. 3 and 4 is an inverted coplanar organic thin film transistor. FIG. 3 does not illustrate a gate electrode, which will be described later.
Referring to FIGS. 3 and 4, in the organic thin film transistor, a gate electrode 12 is formed on a substrate 10 and a gate insulating layer 16 is formed on the entire surface of the substrate 10 to cover the gate electrode 12. Source and drain electrodes 13 and 14 are formed on the gate insulating layer 16. An organic semiconductor layer 15 contacting the source and drain electrodes 13 and 14 is formed.
In the structure described above, as shown in FIG. 3, since the organic semiconductor layer 15 is formed in a circular shape, there are side portions of the source and drain electrodes 13 and 14 that do not contact the organic semiconductor layer 15. Namely, there are non-contact portions in the source and drain electrodes 13 and 14 to the organic semiconductor layer 15. Therefore, when forming an array of organic thin film transistors, the lengths of portions of the source and drain electrodes 13 and 14 contacting the organic semiconductor layer 15 are different between the plurality of organic thin film transistors. As a result, the physical characteristics of the organic thin film transistors, such as threshold voltage, are not uniform. Therefore, in a flat panel display apparatus having a plurality of organic thin film transistors, it is difficult to accurately and clearly reproduce input image signals because the characteristics of the organic thin film transistors are different among the plurality of organic thin film transistors electrically connected to various pixels.